Current antenna systems can be divided into three main categories: 1) antennas that radiate with a fixed pattern and polarization (“standard antennas”), 2) antennas including a matrix of active elements that radiate with variable pattern and/or polarization by appropriately phasing each active element (“phased array”), and 3) antennas including a single active element, showing a different pattern and polarization, depending upon the adopted current distribution on the radiating element (“reconfigurable antennas”). Phased arrays and reconfigurable antennas have received significant attention in the literature with respect to standard antennas thanks to their capability of dynamically changing their radiation properties in response to the multi-variate behavior of the wireless channel. The reconfigurable antenna is preferable over a phase array antenna mainly because it employs a single active element and thus occupies a small space. Reconfigurable antennas also allow for high radiation efficiency since they do not employ phase shifters and power dividers. Reconfigurable antennas also can adapt their characteristics in response to the behavior of the wireless channel and can be used for a variety of applications including throughput maximization, interference management, directional networking, and security.
Various types of reconfigurable antennas capable of changing pattern and polarization have been proposed in the literature. These antennas may employ embedded switches or variable capacitors to change the current distribution on the metallization of the active element or may employ an active antenna element surrounded by passive elements (i.e., parasitic elements) loaded with variable capacitors or connected to switches.
Particularly interesting is the design of Composite Right/Left-Handed (CRLH) Reconfigurable Leaky-Wave Antennas (LWAs), a two-port metamaterial-based design that is able to steer its directive beam from broadside to backward and forward angles. Leaky-wave antennas are based on the concept of traveling-wave, as opposed to conventional resonating-wave behavior. When an RF signal is applied to the input port, the traveling wave progressively “leaks” power as it travels along the waveguide structure. LWAs can also be seen as a phased array traveling wave antenna with amplitude decaying excitation and progressive phase shift as a result of the wave traveling along each unit cell. This leakage phenomenon is directly related to the directivity of the radiated beam.
The CRLH-LWA is a periodic structure made by a cascade of metamaterial unit cells, as shown in FIG. 1. FIG. 1 illustrates the CRLH-LWA design introduced by Patron et al. in “Improved Design of a CRLH leaky-wave antenna and its application for DoA Estimation,” Proc. IEEE-APS Topical Conference on Antennas and Propagation in Wireless Communications (APWC), pp. 1343-1346, September 2013. The design of the single unit cell is appropriately tuned for the propagation constant β to operate within the radiated region of the dispersive curve, which means β<k0 where k0 is the free space wavenumber. By populating the unit cell with varactor diodes in series and shunt configuration, the propagation constant β of the waveguide can be electronically changed through two DC voltages (VS, VSH) from left-hand (LH) to right-hand (RH). This variation of the propagation constant β determines the angle of the radiated main beam. As illustrated in FIG. 2, the antenna consists of a cascade of several unit cells. The CRLH behavior is determined by designing the unit cell to have proper series capacitance and shunt inductive component by means of a shunt microstrip stub. The series capacitance can be varied through two varactor diodes DS1 and DS2, while the shunt component is varied through the varactor DSH. A capacitor (C) is added to the shunt stub in order to decouple the two bias voltages VS and VSH. Thus, these two bias voltages will modulate the propagation constant β along the waveguide and provide the beam steering. Unlike previous LWA designs, this LWA antenna avoids the use of interdigitated capacitors as part of the CRLH equivalent model. As a result, the manufacturing challenges that may be introduced by etching the very thin fingers that constitute the interdigital capacitors are avoided. Consequently, enhanced symmetry between the two input ports is achieved and it also opens a new venue for miniaturization of such a design. The form factor of the antenna can also be reduced by designing the DC lines with spiral and folded RF chokes as well as using lumped elements (L). Prior art designs used long quarter-wavelength transformers for DC biasing.
Although the planar and compact form factor of such LWAs make them suitable for wireless base stations, they conventionally cannot be exploited on mobile devices due to size constraints. The present invention addresses this limitation by presenting an approach that will make LWAs more suitable for mobile devices.
Current attempts to miniaturize antenna dimensions involve the use of non-conventional substrates with high or enhanced dielectric constant. Other techniques were developed where the substrate is made by stacking reactive/magnetic layers. Unfortunately, these techniques introduce more manufacturing complexity and bulk.
On the other hand, recent developments in defected ground structures have shown the possibility of simply properly etching the ground plane of transmission lines or antennas in order to change their cut-off and resonant frequencies. As a result, devices with small dimension can be loaded with complementary split-ring resonators (CSRRs) on the ground plane to resonate at lower frequencies, achieving miniaturization. However, conventional broadside antennas loaded with CSRR for miniaturization suffer from significant back-lobe radiations, thus degrading the front-to-back ratio of the broadside radiation. See, e.g., Sharawi, et al., “A CSRR Loaded MIMO Antenna System for ISM Band Operation,” IEEE Transaction on Antennas and Propagation, Vol. 61, N. 8, August, 2013; Cheng, et al., “A compact omnidirectional self-packages patch antenna with complementary split-ring resonator loading for wireless endoscope application,” Antennas and Wireless Propagation Letters, IEEE 10: 1532-1535, 2011; Pei, et al., “Miniaturized triple-band antenna with a defected ground plane for WLAN/WiMAX applications,” Antennas and Wireless Propagation Letters, IEEE 10: 298-301, 2011; and Xie, et al., “A novel dual-band patch antenna with complementary split ring resonators embedded in the ground plane,” Progress in Electromagnetics Research Letters, Vol. 25, pp. 117-126, 2011. In such systems, the defected ground structure created by the CSRR causes a leakage of the radiation pattern through the ground plane so as to generate a higher amplitude of the back lobe.
Others applications of metamaterials in the art uses split-ring resonators and complementary split-ring resonators for designing transmission lines, filters, and other applications where an electromagnetic wave is propagated through a circuit. In these applications, the transmission lines and filters can be miniaturized or specific performance can be achieved by etching CSRRs underneath the main transmission line. However, filters and transmission lines are used to propagate RF energy, while reconfigurable leaky-wave antennas are used to radiate the energy toward controllable angles. In other applications, split-ring resonators are used to design frequency-reconfigurable antennas where the split-ring resonators are used as resonating elements on the radiating side of the antennas. Such approaches are not used to provide miniaturization techniques for reconfigurable leaky-wave antennas so that such antennas may be used on mobile devices and the like where small size is a significant requirement.